Electromagnetic valve are used in many systems, for example, in a hydraulic oil pressure circuit for controlling oil pressure in an automatic transmission of a vehicle. A current control circuit for the electromagnetic valve, for example, a linear solenoid valve, is disclosed in JP 2014-197655A (US 2014/0254058 A1).
This current control circuit is configured to control an excitation current of the electromagnetic valve by a pulse width modulation signal (PWM signal). The current control circuit periodically varies the excitation current with a predetermined dither amplitude and a dither period longer than a PWM period, which is a pulse period of the PWM signal. The current control circuit thus causes a spool to slightly vibrate in the electromagnetic valve thereby suppressing degradation and variation of responsiveness, which are caused by static friction.
For improving responsiveness, it is proposed to provide the electromagnetic valve in the oil pressure circuit and feed back an output oil pressure of the electromagnetic valve to the spool. According to the proposed control, the spool is biased in a direction of movement by the feedback oil pressure to correct communication between an input port and an output port of the electromagnetic valve.
However, the feedback time for the spool varies with the output oil pressure, length of hydraulic flow circuit, viscosity of oil (temperature of oil) and the like. For this reason, the electromagnetic valve occasionally resonates when the time of feedback of the oil pressure to the spool and the time of small vibration of the spool match. This resonance is a self-excited vibration. The self-excited vibration abrades the spool rapidly.
Even in case that the dither period and the dither amplitude are designed not to cause the self-excited vibration, the self-excited vibration is still likely to arise because of unexpected environmental changes such as oil temperature change, which arise in the course of control operation.